Containers of a polyester as represented by polyethylene terephthalate (PET) feature excellent properties such as transparency, heat resistance, gas-barrier property and the like properties, and have been extensively used for a variety kinds of applications.
In recent years, on the other hand, it is amounting demand to reutilize the resources inclusive of the above polyester containers. Therefore, attempts have been made to recover the used containers and to reuse them as the recycled resin for various applications. Here, many of the contents contained in the packing containers are subject to be degraded with light, as represented by some kinds of beverages, medicines, cosmetics and the like. Therefore, these kinds of contents are provided being contained in opaque containers that are formed by using a resin composition of a resin blended with a coloring agent such as pigment. From the standpoint of reutilizing the resources, however, adding the coloring agent is not desired (since it makes it difficult to maintain transparency in the recycled resins). Therefore, it has now been demanded to use transparent containers. Accordingly, even those opaque containers suited for containing photo-degradable contents must be improved for their properties so as to be used again.
In order to impart light-shielding property (opaqueness) without using coloring agent, it can be contrived to realize a foamed container by making bubbles present in the container wall. A variety of proposals have been made concerning such foamed plastic containers. For instance, a patent document 1 discloses a formed article having a feature in that the foamed cells are having an average diameter that is changing, i.e., the average diameter of the foamed cells present on the front surface side is smaller than the average diameter of the foamed cells present in the inside.
With the formed body in which the average diameter of the foamed cells is changing as proposed by the patent document 1, however, a non-foamed portion (where no foamed cell is present) is thickly formed in the central portion thereof, and light-shielding property is not attained to a sufficient degree. In this case, it can be contrived to form the foamed cells distributed over the whole wall portions so that the non-foamed portion will not be thickly formed in the central portion. According to this means, however, foamed cells of very large sizes are distributed in the central portion of the body portion causing the strength to be very decreased due to the foaming and making it difficult to put it into practical use.
A patent document 2 filed by the present applicant is proposing a foamed plastic container solving the above-mentioned problem. The container of the patent document 2 has a container wall comprising a plastic material in which foamed cells are distributed, the foamed cells in the container wall in the direction of surface thereof having a length that decreases from the outer surface of the container toward the inner surface thereof. Namely, in the patent document 2, a decrease in the gas-barrier property is suppressed by decreasing the size of the cells formed on the inner surface side.
In the container of the patent document 2, however, the size of the foamed cells decreases progressively toward the inner surface side of the container still leaving a problem of a decrease in the light-shielding property despite the foamed cells are formed. Specifically, in a bottle obtained by stretch-molding (blow-molding) the preform, the wall of the body portion has a decreased thickness and the foamed cells overlap less frequently in the direction of thickness. Therefore, multiplex reflection is not so much attained by the foamed cell-containing portion, and the light-shielding property tends to decrease conspicuously. If foaming is promoted to compensate for it, then the bubbles become coarse on the outer surface side making it difficult to attain a high mirror gloss and, therefore, leaving room for improvements.
The present applicant has proposed in a patent document 3 a foamed plastic container in which foamed cells of a flat shape are distributed in the wall being overlapped one upon the other in the direction of thickness thereof, the foamed cells having an average long diameter of not more than 400 μm and an average aspect ratio (long diameter/short diameter) of not less than 6 as viewed in cross section of the container wall in a direction in which it is stretched to a maximum degree. The container has such advantages as pearl-like appearance and a high commercial value. Like the patent document 2, however, the plastic container of the patent document 3, too, still has room for improvements in regard to that it cannot attain a high light-shielding property together with a high mirror gloss if the portion where the foamed cells are formed is forced to have only a limited thickness like that of a bottle. That is, either the patent document 2 or 3 has the foamed cells of a flat shape but also has a problem in that the mirror gloss is lost as the number of the foamed cells increases.